Field of the Invention
The present invention relates to a wireless power transfer technique.
Description of the Related Art
There is known a wireless power transfer system which includes a power transmitting apparatus for wirelessly transmitting power and a power receiving apparatus for receiving power supplied from the power transmitting apparatus and in which wireless power transfer is performed from the power transmitting apparatus to the power receiving apparatus. The wireless power transfer method broadly includes three kinds of methods, that is, the electromagnetic induction method, the microwave transfer method, and the resonant magnetic coupling method.
The basic principle of the electromagnetic induction method is to make an inductive current run in a power receiving coil by causing a magnetic flux which is generated from running a current in a power transmitting coil to penetrate the power receiving coil. Therefore, in the electromagnetic induction method, the positions of the power transmitting coil and the power receiving coil are generally adjusted so that the magnetic flux generated by the power transmitting coil effectively penetrates the power receiving coil.
In the microwave transfer method, the power transmitting apparatus emits wireless power in the air by an antenna and the power is received by the antenna of the power receiving apparatus. Since power loss in the air increases in the microwave transfer method, power transfer is generally performed in a specific direction by increasing the directionality of the antenna.
In the resonant magnetic coupling method, power transfer is performed by causing the power transmitting apparatus and the power receiving apparatus to resonate at the same frequency. Power transfer (with at least sufficient power) cannot be performed unless the power transmitting apparatus and the power receiving apparatus can resonate at the same frequency. Japanese Patent Laid-Open No. 2010-063245 and Japanese Patent Laid-Open No. 2012-518381 disclose methods that allow power to be transmitted selectively to one power receiving apparatus as a power transmission target out of a plurality of power receiving apparatuses by using this feature to cause the power transmitting apparatus to change the resonance frequency. Note that in the case of the resonant magnetic coupling method, since it is sufficient for the power transmitting apparatus and the power receiving apparatus near there to resonate at the same frequency, the method is not sensitive to positional accuracy compared to the electromagnetic induction method and cannot ensure that a transfer efficiency will be high only in the desired direction as in the case of using microwaves.
The resonant magnetic coupling method wireless power transfer system is an effective method for selectively transferring power to a plurality of power receiving apparatuses when the resonance frequencies are different and the resonance frequency bandwidths are narrow. The transfer efficiency of power transmission/reception using the resonant magnetic coupling method depends on a coupling coefficient ki between the power transmitting circuit and the impedance of the power transmitting antenna, a coupling coefficient ku between the antennas at the non-load time which is influenced by the distance between and the sizes of the antennas, and a coupling coefficient ko between the power receiving antenna and the impedance of the power receiving circuit.
The maximum transfer efficiency is obtained when ku≥√(ki×ko), and especially when ku=√(ki×ko), the resonance frequency between the antennas match with the resonance frequency f0 of each single antenna. This state is called critical coupling. When the distance between the power transmission/reception antennas is short and ku>√(ki×ko), two resonance frequencies that are lower and higher, respectively, than the resonance frequency f0 will be generated between the power transmission/reception antennas. This state is called tight coupling. On the other hand, when the distance between the power transmission/reception antennas is long and ku<√(ki×ko), the resonance point (resonance frequency) of the power transmission/reception antennas is in the same state as critical coupling, but the coupling efficiency decreases. This state is called loose coupling.
FIGS. 12A and 12B each schematically show the relationship between the coupling efficiency and the frequency in the cases of tight coupling, critical coupling, and loose coupling. As shown in FIG. 13, note that in the actual environment of usage, by causing the coupling to become closer to tight coupling than critical coupling, an adjustment can be performed to prevent the efficiency from decreasing even if the distance to the antenna incorporated in the power receiving apparatus more or less shifts, and thus maintain the efficiency. However, if the adjustment as shown in FIG. 13 is made, since the frequency band with high transfer efficiency between the antennas is wide, resonance occurs at a wide frequency band.
In such a case, if power transfer from a second power transmitting apparatus to a second power receiving apparatus is started while power is transferred between a first power transmitting apparatus and a first power receiving apparatus, the power transmitted from each power transmitting apparatus can be received by an apparatus that is not the power receiving apparatus which is the original partner apparatus of the power transfer. That is, power transmitted from the first power transmitting apparatus can be received by the second power receiving apparatus, and the power transmitted from the second power transmitting apparatus can be received by the first power receiving apparatus. In such a case, there is a possibility that the originally intended power transmission/reception cannot be performed.
The present invention provides a technique that prevents transmitted power from being received by an apparatus that is different from the target apparatus of power transmission.